To build the Car-glass timer, you need
lots of cars. These become the particles
of
sand in your appropriately gigantic timer.

Instead of slipping through the gap
between the top and bottom container,
where they might jam up, there is a
double action rotating gate mechanism
which releases
one car at a time. Once
the
bottom gate opens, a single car drops
down the hundreds of feet into the lower
chamber, where it smashes against the
bottom of the toughened glass. Cars
which don't totally make it through the
two gates are simply severed into
segments by the guillotine action of the
closing gate.

That's all there is to it, apart from
equipping it with an engine powerful
enough to crank the mechanism and
invert it, once all the cars have dropped
through the gated orifice separating the
top from the bottom holding vessels.

In time all of the cars will be reduced to
smaller and smaller particles, eventually
becoming no bigger than the grains of
sand they initially represent.